Ghost nets in the Columbia River: Methods for locating and removing derelict gill nets in a large river and an assessment of impact to white sturgeon

Lost gill nets, known as ghost nets, represent a threat to aquatic species. Gill nets have been in use in the Columbia River commercial fishery since the mid-1800s, and a number of gill nets continue to be lost each year. We designed a study to investigate the effect of lost nets in the Columbia River. For this study, we developed methods to recover nets and evaluated the use of side-scan sonar (SSS) to locate nets. Grappling in known commercial fishing areas effectively removed nets and was more efficient than relying on SSS to direct recovery efforts. In 154 grapple tows, we recovered 33 nets containing 126 white sturgeon Acipenser transmontanus. Newly lost nets caught significantly more white sturgeon than older nets. Recovered nets at large less than I year were responsible for 63% of the total catch, nets at large 1-4 years captured 24%, and nets at large more than 4 years captured 13%. We determined that approximately 10 nets are lost each year, estimated that lost nets have a life span of approximately 7 years, and estimated that ghost nets may kill more than 545 white sturgeon annually. White sturgeon managers may wish to consider developing an outreach program directed at reducing net loss and a net removal program.